1. Field of the Invention
The present invention relates to a semiconductor device having a circuit composed of a thin film transistor (hereinafter referred to as TFT) and a method of manufacturing the same. In particular, the present invention relates to an electro-optical device typified by a liquid crystal display device, a semiconductor device mounted with the electro-optical device as its component and a method of manufacturing the semiconductor device. Note that, the semiconductor device in this specification indicates devices in general being capable of functioning with the use of semiconductor characteristics, and electro-optical devices, semiconductor circuits and electronic equipment are all included in the semiconductor devices.
2. Description of the Related Art
In recent years, the development of a semiconductor device including a large-area integrated circuit formed from a TFT-constituted of a thin film (thickness of approximately several to several hundreds of nm) formed on a substrate having an insulating surface has been progressing. As typical examples of the semiconductor device, an active matrix liquid crystal display device and a light emitting device are known. In particular, since the TFT in which a crystalline silicon film is used for an active region has high electric field mobility, it can constitute various function circuits.
For example, in the active matrix liquid crystal display device a pixel circuit for conducting image display for each functional block and a driver circuit, which is mainly composed of a CMOS circuit, for controlling the pixel circuit, such as a shift register circuit, a level shifter circuit, a buffer circuit or a sampling circuit, are formed on one substrate.
Further, the TFT comprises at least a semiconductor film, an insulating film constituted of a silicon oxide film, a silicon oxynitride film or the like, and wirings constituted of various metal materials. The wirings include a source wiring, a gate wiring (including a gate electrode), and the like. These films each have a thickness of approximately several to several hundreds of nm, and thus, can be said to be thin films.
These thin films are formed by known film formation techniques such as CVD (chemical vapor phase growth method) and sputtering. However, it is known that the thin film has an internal stress. Note that the internal stress includes an intrinsic stress and a thermal stress that arises from a difference in a thermal expansion coefficient between the thin film and a substrate.
The influence of the thermal stress can be disregarded by taking a material for a substrate, a process temperature, a pressure and the like into consideration. However, the generation mechanism of the intrinsic stress has not been absolutely made clear. It is rather considered that the intrinsic stress is generated due to a phase change and a composition change which are complicatedly interwound with each other and caused in a film growth process or by the subsequent heat treatment.
The internal stress generally includes a compressive stress and a tensile stress. As shown in FIG. 5A, when a thin film 311 is to expand, a substrate 312 is compressed and formed such that the thin film 311 is on the outside thereof. This is called the compressive stress. On the other hand, as shown in FIG. 5B, when the thin film 311 is to contract with respect to the substrate 312, the substrate 312 pulls the thin film 311 in a direction in which the contraction is hindered so that the substrate 312 deforms such that the thin film is on the inside thereof. This is called the tensile stress. In general, the value of the tensile stress is shown with “+”, and the value of the compressive stress is shown with “−” in many cases.
The influence of the internal stress described above on electric characteristics of a transistor is described in, for example, “Influence of Stress of Etch Stop Nitride Film to 0.13 μm CMOS Transistor Performance; Applied Physics Society Silicon Technology Section No. 25 Special Number on ULSI Device (2001) pp 36-39.” According to this, it is reported that the mobility of an NMOS transistor is enhanced when a channel forming region thereof receives a tensile stress while the mobility of a PMOS transistor is enhanced when a channel forming region thereof receives a compressive stress.
As described above, a wiring of a TFT is formed from a thin film. Therefore, the wiring also has an internal stress, and there has been a case where peeling is is generated if the internal stress is strong. Further, a gate electrode formed from the same material as that for the wiring is formed on a semiconductor film through an insulating film. The internal stress of the gate electrode acts on even the semiconductor film, and imparts distortion to an interface between the insulating film and the semiconductor film or the semiconductor film. Thus, there is a case where electric characteristics typified by a threshold voltage and electric field mobility are affected.